The present invention relates to a tabular grain having a very small thickness, a high aspect ratio and a large equivalent-circle diameter; a silver halide emulsion comprising the tabular grain; a process for producing the silver halide emulsion; and a novel crystal phase-controlling agent compound suitable for the production of the tabular grain.
The tabular silver halide grain (hereinafter referred to as a xe2x80x9ctabular grainxe2x80x9d) has the following photographic properties:
1) the ratio of surface area to volume (hereinafter referred to as a xe2x80x9cspecific surface areaxe2x80x9d) is large and a large amount of sensitizing dye can be adsorbed to the surface of the grain, so that the color sensitization sensitivity can be relatively high as compared with the intrinsic sensitivity;
2) when an emulsion containing tabular grains is coated and dried, the grains are oriented in parallel to the support surface, so that the coated layer can be reduced in the thickness and the photographic light-sensitive material obtained can have good sharpness;
3) in an X-ray photographic system, when a sensitizing dye is added to the tabular grain, the silver halide cross-over light can be extremely reduced and therefore, the deterioration of image quality can be prevented;
4) light scattering is reduced and therefore, an image of high resolution can be obtained; and
5) the sensitivity to blue light is low, so that when the tabular grain is used in a green-sensitive layer or a red-sensitive layer, a yellow filter can be removed from the emulsion.
By virtue of these advantageous properties, tabular grains have been heretofore used in commercially available light-sensitive materials.
JP-B-6-44132 (the term xe2x80x9cJP-Bxe2x80x9d as used herein means an xe2x80x9cexamined Japanese patent publicationxe2x80x9d) and JP-B-5-16015 disclose a tabular grain emulsion having an aspect ratio of 8 or more. The aspect ratio as used herein means a ratio of the equivalent-circle diameter to the thickness of a tabular grain. The diameter of a grain as used herein means the diameter of a circle having an area equal to the projected area of a grain when the emulsion is observed through a microscope or an electron microscope. The thickness is shown by the distance between two parallel main surfaces constituting a tabular silver halide.
JP-B-4-36374 discloses a color photographic light-sensitive material which is improved in the sharpness, sensitivity and graininess by using tabular grains having a thickness of less than 0.3 xcexcm and a diameter of 0.6 xcexcm or more in at least one layer of green-sensitive emulsion layer and red-sensitive emulsion layer.
In recent years, with the progress of silver halide light-sensitive materials designed to have higher sensitivity and smaller format, a color light-sensitive material having higher sensitivity and improved image quality is keenly demanded. To meet this requirement, the silver halide emulsion is demanded to have higher sensitivity and more excellent graininess. Conventional tabular silver halide emulsions cannot cope with these requirements and more improvement of the performance is demanded.
As the aspect ratio of a tabular grain is larger, the specific surface area is larger and the above-described advantageous properties of a tabular grain can be more effectively utilized. In other words, a larger amount of a sensitizing dye is adsorbed to a larger surface area and a larger intensity of light is absorbed per one grain, whereby higher sensitivity can be obtained. Therefore, many studies have been heretofore made to prepare tabular grains reduced in the thickness. JP-B-5-12696 discloses a method of oxidizing and thereby ineffectuating a methionine group in gelatin and preparing thin tabular grains using the gelatin as a dispersion medium, JP-A-8-82883 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a method of ineffectuating the amino group and the methionine group in gelatin and preparing thin tabular grains using the gelatin as a dispersion medium, and JP-A-10-148897 discloses a method of chemically modifying the amino group in gelatin to introduce at least two or more carboxyl groups and preparing thin tabular grains using the gelatin as a dispersion medium.
As for a tabular grain having a very large equivalent-circle diameter and a very small thickness, U.S. Pat. No. 5,612,175 discloses the tabular grain. In Examples of this patent, silver bromide tabular grains having a thickness of less than 0.1 xcexcm and an average equivalent-circle diameter of several xcexcm are disclosed. In the Examples, the thinnest tabular grain had a thickness of 0.04 xcexcm and an average equivalent-circle diameter of 3.9 xcexcm. European Patent 896,245 discloses, in the Example, a tabular grain having a thickness of 0.048 xcexcm and an average equivalent-circle diameter of 3.74 xcexcm. The tabular grains disclosed in these patents are expected to ensure high sensitivity and good graininess because of their very small thickness and very large main surfaces, however, in order to satisfy the recent requirement for higher sensitivity, tabular grains are still demanded to have a smaller thickness and larger main surfaces.
A crystal phase-controlling agent is an effective material for forming thin tabular grains, however, only with known compounds, the demanded aspect ratio cannot be achieved. For obtaining yet higher sensitivity, a crystal phase-controlling agent having higher performance, namely, a crystal phase-controlling agent having higher selectivity for the (111) face of a silver halide grain is being demanded. On the other hand, the crystal phase-controlling agent itself disadvantageously inhibits the adsorption of a sensitizing dye or chemical sensitization and therefore, is preferred not to be present on a tabular grain at the time of adding a sensitizing dye or at the chemical sensitization. From these reasons, a step of removing the crystal phase-controlling agent by water washing or the like is generally provided after the grain formation or before or after the adsorption of a sensitizing dye, however, the crystal phase-controlling agent cannot be completely removed and at present, this problem cannot be satisfactorily solved. In this meaning, a crystal phase-controlling agent which can be easily removed after the grain formation is demanded.
It is a well-known fact in the art that a silver halide emulsion is gradually sensitized due to naturally occurring radiation and with the passage of a long time, fogging is already generated before the exposure for the image recording and seriously deteriorates the image quality obtained by the exposure. In a silver halide emulsion, a latent image is formed on a silver halide grain upon exposure and thereby an image is recorded. For achieving high sensitivity, the size of silver halide grain must be increased to elevate the intensity of light absorbed at the exposure. On other hand, the absorption of naturally occurring radiation is, as well known, proportional to the volume of a silver halide. Thus, a silver halide emulsion may have higher sensitivity but at the same time, suffers from the increase of fogging due to radiation. Many efforts have been heretofore made to solve this problem but a definite method for solving this problem is not yet discovered. For solving this difficult problem, the effective means is to realize a grain having a very small thickness and a very large main surface area. More specifically, it is obvious that the intensity of light absorbed into a silver halide at the recording of an image is proportional to the amount of a sensitizing dye adsorbed to the surface of a silver halide grain and the amount of a sensitizing dye is proportional to the surface of a silver halide grain. On the other hand, the absorption of radiation is, as described above, proportional to the volume of a silver halide.
Accordingly, the tabular grain having a very large main surface area and a very small thickness and thereby having a very small grain volume, disclosed in the present invention, can solve the above-described difficult problem.
A first object of the present invention is to provide a silver halide emulsion comprising tabular grains, in which the tabular grain has a very small thickness and the main surface thereof is a (111) face and has a very large surface area. A second object of the present invention is to provide a crystal phase-controlling agent which can ensure the formation of silver halide grains having a high aspect ratio and which can be easily removed after the grain formation. A third object of the present invention is to provide a silver halide grain having a high aspect ratio, which is formed using the crystal phase-controlling agent, and a process for producing a silver halide emulsion containing the silver halide grain.
The above-described objects of the present invention can be attained by the following means.
(1) A silver halide emulsion comprising light-sensitive silver halide grains having a silver bromide content of 70 mol % or more, with 60% or more of the entire projected area of the silver halide grains being occupied by tabular grains having an average grain thickness of less than 0.04 xcexcm, an average equivalent-circle diameter of 4 xcexcm or more, and (111) face as main surfaces.
(2) The silver halide emulsion as described in (1), wherein 75% or more of the entire projected area of the silver halide grains is occupied by tabular grains having an average grain thickness of less than 0.04 xcexcm, an average equivalent-circle diameter of 4 xcexcm or more, and (111) face as main surfaces.
(3) The silver halide emulsion as described in (1), wherein 90% or more of the entire projected area of the silver halide grains is occupied by tabular grains having an average grain thickness of less than 0.04 xcexcm, an average equivalent-circle diameter of 4 xcexcm or more, and (111) face as main surfaces.
(4) A method for producing the silver halide emulsion described in (1), (2) or (3), comprising nucleation, ripening and growth steps and performing these steps while letting at least one compound represented by the following formula (I), (II) or (III) be absent at the time of nucleation and be present at the time of ripening and growth to obtain an emulsion comprising tabular grains, wherein a mixing vessel is separately provided from a reactor for performing the nucleation and/or growth of silver halide grains, an aqueous solution of a water-soluble silver salt and an aqueous solution of a water-soluble halide are fed to the mixing vessel and mixed to form silver halide fine grains, and the fine grains are immediately fed to the reactor to perform the nucleation and/or growth of silver halide grains in the reactor: 
wherein R1 represents an alkyl group, an alkenyl group or an aralkyl group, R2, R3, R4, R5 and R6 each represents a hydrogen atom or a substituent, each of the pairs R2 and R3, R3 and R4, R4 and R5, and R5 and R6 may form a condensed ring, provided that at least one of R2, R3, R4, R5 and R6 represents an aryl group, and Xxe2x88x92 represents a counter anion; 
wherein A1, A2, A3 and A4, which may be the same or different, each represents a nonmetallic atom group for completing the nitrogen-containing heterocyclic ring, B represents a divalent linking group, m represents 0 or 1, R1 and R2 each represents an alkyl group, X represents an anion, and n represents 0, 1 or 2, provided that when an inner salt is formed, n is 0 or 1.
(5) The method for producing the silver halide emulsion as described in (1), (2) or (3), comprising nucleation, ripening and growth steps and performing a part or the whole of the growth step in the presence of an alkali-treated ossein gelatin containing 30% or more of a xcex3 component and a component having a molecular weight higher than that of the xcex3 component, wherein a mixing vessel is separately provided from a reactor for performing the nucleation and/or growth of silver halide grains, an aqueous solution of a water-soluble silver salt and an aqueous solution of a water-soluble halide are fed to the mixing vessel and mixed to form silver halide fine grains, and the fine grains are immediately fed to the reactor to perform the nucleation and/or growth of silver halide grains in the reactor.
(6) The method for producing a silver halide emulsion as described in (4) or (5), wherein an esterified gelatin is used as a protective colloid for forming the silver halide fine grains in the mixing vessel.
(7) A silver halide emulsion comprising a compound represented by formula (IV) or (V): 
wherein R10, R11, R12, R13 and R14 each represents a hydrogen atom or a substituent, Y1, Y2, Y3, Y4 and Y5 each represents a hydrogen atom or a substituent, provided that at least one of Y1, Y2, Y3, Y4 and Y5 is a group selected from the group consisting of xe2x80x94SO2NH2, xe2x80x94SO2NHR15, xe2x80x94SO2N(R15)2, xe2x80x94SO3xe2x88x92, xe2x80x94CONH2, xe2x80x94CONHR15, xe2x80x94CON(R15)2, xe2x80x94NHSO2NH2, xe2x80x94NHSO2NHR15, xe2x80x94NHSO2N (R15)2 and xe2x80x94SO2NHCOR15, Xxe2x88x92 represents a counter anion, na represents a number necessary for neutralizing the electric charge of the compound, and R15 represents a substituted or unsubstituted alkyl, alkenyl, alkynyl or aryl group; 
wherein A represents an organic residue for completing the nitrogen-containing aromatic heterocyclic ring and A""s may be the same or different, L1 and L2 each represents a divalent linking group, Y represents xe2x80x94C(xe2x95x90O)xe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94NHC(xe2x95x90O)xe2x80x94 or xe2x80x94NHC(xe2x95x90S)xe2x80x94, Xxe2x88x92 represents a counter anion, and na represents a number necessary for neutralizing the electric charge of the compound.
(8) A silver halide grain emulsion obtained by forming the grains in the presence of a compound represented by formula (IV) or (V).
(9) A method for producing a silver halide grain emulsion, comprising performing a part or the whole of the grain formation step in the presence of a compound represented by formula (IV) or (V) to produce a silver halide emulsion comprising tabular grains having (111) face as main surfaces.
(10) The method for producing the silver halide emulsion as described in (4), wherein the compound represented by formula (I), (II) or (III) is a compound represented by formula (IV) or (V) described in the above item (7).
(11) A pyridinium compound represented by formula (IV).
(12) The pyridinium compound as described in (11), wherein in formula (IV), R10 to R14 each is a hydrogen atom or an alkyl group having from 1 to 3 carbon atoms and Y1 to Y5 each represents a hydrogen atom or xe2x80x94SO2NH2.
(13) A silver halide photographic light-sensitive material comprising the silver halide emulsion described in (1) to (11) or a silver halide emulsion produced by the production process described in (1) to (11).